U.S. patent application number 14/256419 was filed with the patent office on 2015-10-22 for developer free positive tone lithography by thermal direct write.
The applicant listed for this patent is Massachusetts Institute of Technology. Invention is credited to Pao Tai Lin, Jonathan Phillip Singer, Edwin Lorimer Thomas.
Application Number | 20150303064 14/256419 |
Document ID | / |
Family ID | 50771650 |
Filed Date | 2015-10-22 |
United States Patent
Application |
20150303064 |
Kind Code |
A1 |
Singer; Jonathan Phillip ;
et al. |
October 22, 2015 |
Developer Free Positive Tone Lithography by Thermal Direct
Write
Abstract
A method for lithographic patterning of thin films. A thin film
is deposited on a substrate and the film is exposed to optical
energy from a focused laser to induce a thermal gradient in the
film by optical absorption. The film is softened through a melting
or glass transition process and the thermal gradient induces a
directional dewetting down the thermal gradient. The invention
permits developer free positive tone lithography by thermal direct
write and also metrology of the thin film by the morphology of the
resultant features.
Inventors: |
Singer; Jonathan Phillip;
(Cambridge, MA) ; Lin; Pao Tai; (Brighton, MA)
; Thomas; Edwin Lorimer; (Houston, TX) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Massachusetts Institute of Technology |
Cambridge |
MA |
US |
|
|
Family ID: |
50771650 |
Appl. No.: |
14/256419 |
Filed: |
April 18, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61814889 |
Apr 23, 2013 |
|
|
|
Current U.S.
Class: |
438/694 |
Current CPC
Class: |
G03F 7/70383 20130101;
H01L 21/308 20130101; G03F 1/68 20130101 |
International
Class: |
H01L 21/308 20060101
H01L021/308 |
Goverment Interests
[0002] This invention was made with government support under
contract number W911NF-07-D-0004 awarded by the Army Research
Office. The government has certain rights in the invention.
Claims
1. A method for lithographic patterning of thin films comprising:
depositing a film of less than about 500 nm thickness of material
containing at least one non-metallic component on a substrate;
exposing the film to optical energy from a focused (spot size less
than about 25 .mu.m) laser to induce a thermal gradient in the film
by optical absorption, wherein the film is softened through a
melting or glass transition process and the thermal gradient
induces a directional dewetting along the thermal gradient.
2. The method of claim 1 wherein the thermal gradient arises from
optical absorption of the substrate.
3. The method of claim 1 wherein the thermal gradient arises from
optical absorption of the film.
4. The method of claim 1 wherein the focused laser has a selected
focal shape.
5. The method of claim 4 wherein the selected focal shape is
selected from the group consisting of profiles including Gaussian
or toroidal.
6. The method of claim 1 further including exposures that overlap
or are adjacent to existing features.
7. The method of claim 6 where the overlapped feature is a region
of film of differing thickness to produce antireflection effects to
generate hotspots and dewetting away from main focus.
8. The method of claim 7 where the thickness variation is produced
by a previous exposure.
9. The method of claim 1 wherein the film is healed or cooled on a
length scale greater than the focal spot size to effect a patterned
thermal profile.
10. The method of claim 9 wherein the film is heated or cooled with
a thermoelectric platform.
11. The method of claim 1 wherein the film comprises multiple
layers having different properties.
12. The method of claim 11 wherein the multiple layers dewet at
different temperature dependent rates without mixing.
13. The method of claim 1 further including a film having
nanoparticles incorporated therein.
14. The method of claim 1 wherein the film is exposed to a solvent
vapor to control patterning effects by effects including but not
limited to evaporative cooling and surface smoothing.
15. The method of claim 14 wherein the solvent vapor selectively
swells one or more components of the film.
16. The method of claim 1 where the properties of the underlying
substrate are altered to influence the dewetting behavior.
17. The method of claim 16 where the properties of the underlying
substrate are altered by surface functionalization with a polymer
brush or self-assembled monolayer.
18. The method of claim 1 where the dewetting is incomplete,
leaving the substrate coated by some thickness of the film.
19. The method of claim 1 wherein the film is a polymer or organic
molecule of thickness less than about 100 nm.
20. The method of claim 19 wherein the film is a block
copolymer.
21. The method of claim 2 wherein the substrate is a semiconductor
whose optical energy absorption leads to thermal heating and
thermal gradient.
22. The method of claim 1 where the relative position of the
focused spot is moved with respect to the film by either motion of
the spot or motion of the substrate.
23. The method of claim 1 further including using the patterned
film as a plasma etch mask for the substrate.
24. The method of claim 1 where the morphology of the dewetted
feature is used to determine the properties of the film.
Description
[0001] This application claims priority to provisional application
Ser. No. 61/814,889 filed Apr. 23, 2013, the contents of which are
incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0003] This invention relates to lithography and more particularly
to developer free positive tone lithography by thermal direct
write.
[0004] Laser spike annealing (LSA) is a prior art alternative to
standard thermal treatment in semiconductor technology..sup.1 In
this technique, a high intensity continuous wave (CW) or pulse
laser is rapidly scanned across an absorbing surface, such as a
silicon wafer. As semiconductor materials generally possess high
thermal conductivities, the local temperature at the laser spot
spikes to a high value and then, once the laser light is removed,
very rapidly drops back to ambient temperature. As a result, both
the temperature and annealing time can be precisely controlled by
selection of laser intensity and exposure time. Additionally,
annealing can be performed while kinetically avoiding unwanted
effects, such as diffusion of the gates. More recently LSA has been
applied to the annealing of soft materials for the phase separation
of block copolymers (BCPs).sup.2 and chemically amplified resist
(CAR) post-baking..sup.3
[0005] An object of the present invention is to turn prior art
broad-focus LSA into a tightly focused direct write technique, as
we have recently reported..sup.4
SUMMARY OF THE INVENTION
[0006] The method according to the invention for lithographic
patterning of thin films includes depositing a film of less than
about 500 nm thickness of material that contains at least one
non-metallic component on a substrate and exposing the thin film to
optical energy from a focused laser to increase film temperature
and to induce a thermal gradient in the film by optical absorption.
The temperature increase softens the film through a melting or
glass transition process and the thermal gradient induces a
directional dewetting down the thermal gradient. In a preferred
embodiment, the increase in film temperature arises from optical
absorption of the substrate. It is preferred but not necessary that
the focused laser have a selected focal shape such as Gaussian or
toroidal.
[0007] Another embodiment includes adjacent or overlapping
exposures to change film thickness to produce antireflection
effects to generate hotspots away from the main focus. The adjacent
or overlapping exposures generate thermal gradients to push
material toward previously formed features in cooler regions.
[0008] In another preferred embodiment, the thin film includes
multiple layers having different properties. It is preferred but
not necessary that the multiple layers dewet at different rates.
Another embodiment includes a resist having nanoparticles
incorporated therein. Another embodiment includes a polymer swollen
in solvent vapor to control thermal effects by evaporative cooling
and results in mobility modification effects, such as smoothing of
patterned films.
BRIEF DESCRIPTION OF THE DRAWING
[0009] FIG. 1 is a simplified schematic diagram of FLaSk
dewetting.
[0010] FIG. 2a is a graph of surface reflectivity versus film
thickness.
[0011] FIG. 2b is a graph of expected peak temperatures (solid) and
thermal gradients (dashed) against power.
[0012] FIG. 3a is an atomic force microscope measurement of an
isolated HSQ dewetted Line with a 2D profile.
[0013] FIG. 3b is an AFM line profile scan illustrating trench
depth.
[0014] FIG. 4 are photomicrographs showing AFM scans of PS and HSQ
lines with various write periodicities.
[0015] FIG. 5 are AFM line profile scans of dewetted PS-PDMS BCP
lines patterned with and without PS-selective solvent vapor.
[0016] FIG. 6a are observed linewidths of PS films of different
molecular weight extracted from AFM scans.
[0017] FIG. 6b are observed linewidths of PS films of different
thicknesses extracted from AFM scans.
[0018] FIG. 7 is a photomicrograph of an AFM scan of a dewetted PS
film derived from a commercial polystyrene cup.
[0019] FIGS. 8a,b are photomicrographs of AFM scans of a dewetted
PS/PVP bilayer forming lines of dots. (b) is patterned a higher
power resulting in deeper features of the same size, but with more
dots per length.
[0020] FIGS. 9a,b are scanning electron micrographs of silicon
patterns produced by etching of dewetted PVP/PVP nanoparticle
films, (a) is patterned at a lower power showing little effect on
the particles (b) is patterned at a higher power showing the
ability to remove the particles with higher temperature
degradation.
[0021] FIG. 10 is a scanning electron micrograph of a PS-PDMS BCP
film with a partially dewetted circular path patterned with solvent
vapor. The BCP microphase (here, cylindrical) is both annealed and
aligned along the direction of the write path. Image made possible
by plasma etching of the PS.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0022] The approach in focused laser spike annealing (FLaSk) (first
introduced for CARs.sup.5) is to turn the broad-focus LSA into a
tightly focused direct write technique. This introduces several new
aspects to the approach with regards to soft matter. The first is
the presence of surrounding confinement--polymers can undergo large
changes in their mechanical properties and dimensions with even
mild increases in temperature, especially if the temperature
crosses the glass transition temperature of the polymer; however in
a FLaSk process, the heated polymer is always surrounded by rigid
polymer at a much lower temperature leading to the potential for
very large mechanical strains. The second, following from the first
is the presence of large (.about.1-100 K/.mu.m) thermal
gradients.
[0023] In the thin (<150 nm) films heated from the substrate (2D
FLaSk), it was observed the films would be selectively removed when
sufficient heat was applied by thermal induced dewetting. This was
observed for polystyrene, polyvinylpyrrolidone, polyvinylacetate,
and polystyrene-polydimethylsiloxane block copolymers, and occurred
regardless of the substrate, though some key features were
affected.
[0024] Dewetting has been utilized as a method to generate
nanopatterns in thin films of metals.sup.5 and polymers.sup.6
through film-stability-based self-assembly, most generally under
near global heat provided by a hotplate or pulsed LSA. In these
techniques the driving force is an instability in the surface
energy of the film with respect to dewetted droplets leading to the
growth of random fluctuations in the film. Recently, a related
technique based on the flow of liquids down a thermal gradient
(thermocapillarity).sup.6-10 has been developed to form large area
nanopillar arrays..sup.11,12 The generalized expression for
thermocapillary force is:
? n ^ = .gamma. T .gradient. T ? indicates text missing or
illegible when filed Eq . 1 ##EQU00001##
where .tau. is the shear, n the surface normal, and .gamma. the
surface tension. The surface tension almost always decreases with
temperature leading to a net force down a thermal gradient. In the
case of the nanopillars, the thermal gradient was generated by
using a heated substrate under a thin film, an air gap, and a
floated cooled superstrate to create sharp (.about.50 K/.mu.m)
thermal gradients, which drives the formation of the pillars. For
the FLaSk technique, the in-plane gradients generated radially from
the spot can be equal to or greater than those generated in the
pillar technique; however, FLaSk provides these gradients on the
micron scale rather than to the whole film at once thus enabling a
DW technique. This is shown schematically in FIG. 1.
[0025] In FIG. 1, a laser 10 is focused on the surface of an
absorbing substrate 12 through a polymer layer 14, generating a
hotspot by thermal absorption. The generated thermal gradient and
induced melting of the polymer lead to radial dewetting that can be
employed in a direct write fashion by translating the relative
position of the focused beam.
[0026] These temperature effects are enhanced by the polymer film
itself which acts as an antireflective coating and thus generates
unusual effects during the dewetting depending on the starting
thickness of the film. For example, as will be seen, adjacent lines
need not all be patterned.
[0027] To understand the mechanism of 2D flask dewetting, the
temperature of the silicon substrate during line writing must be
considered. Determining this is complicated by the fact that the
polymer layer acts as an antireflective coating (ARC) for the
silicon. To determine the effects of such an ARC, the transfer
matrix method is utilized:.sup.13,14
.rho. ij = n i - n j n i + n j Eq . 2 .tau. ij = n i - n j n i + n
j Eq . 3 ? Eq . 4 R = .rho. 12 + .rho. 12 ? 1 + .rho. 22 .rho. 22 ?
2 ? indicates text missing or illegible when filed Eq . 5
##EQU00002##
where i and j are indexes that indicate the layers which the light
is propagating from (i) and to (j) in a particular step, n is the
index of refraction, d is the ARC thickness, .lamda. is the free
space wavelength and R is the total reflection after all three
layers, being air (1), polymer (2), and silicon (3) are considered.
The intermediate values .rho..sub.ij, .tau..sub.ij, and .tau..sub.d
are the reflection, transmission, and phase shift values through
the respective layer pairs or the ARC respectively.
[0028] Taking the polymer layer to be an ARC of index n=1.55 on the
silicon with a nominal starting thickness of 60 nm, the reflection
determined from Eq. 5 is shown in FIG. 2a. With this as a starting
point, FEM simulations were utilized to estimate the temperatures
and gradients that the polymer would experience during the FLaSk
anneal. The thermal profile was analytically modeled as a Gaussian
source (NA=0.4) and the optical absorption and heating that would
be expected to occur. For materials properties,
temperature-dependent values for silicon substrate were utilized
for the thermal conductivity, heat capacity, and density..sup.15
The optical absorption also displays an exponential dependence on
temperature, so it was simulated using a previously derived
empirical model for near-intrinsic silicon excited with 532 nm
light..sup.16 Finally, the starting point for the absorbed power by
the substrate (to be modified by the ARC results) was determined by
measuring the damage threshold (about 350-355 mW), which
corresponds to a peak temperature of .about.1700 K where the
silicon surface melts..sup.17 Using these simulations, it is
possible to plot the range of peak temperatures and thermal
gradients expected for a given power as the film dewets (FIG. 2b).
The expected thermal history can be taken as traces through the
range of temperatures as the film heats by the moving source and
simultaneously thins. This is a highly complex process as the film
will heat faster when it is thicker and thus likely thins before
peak temperature is reached; however peak gradients, which are
spatially in front of the gradients, may be experienced. It can be
seen that for the power range employed (about 200-320 mW) gradients
of around 100.about.1000 K/.mu.m, around an order greater than
those from vertical dewetting, can be expected.
[0029] To achieve a reduction to practice, untreated, silicon
substrates were coated with films of polystyrene (PS), hydrogen
silsesquioxane (HSQ), polyvinylpyrrolidone (PVP), and
polyvinylacetate (PVAc) at various molecular weights to prepare
films of various thicknesses in the range of 50-110 nm. Patterning
was performed with system with a Coherent RegA 532 nm laser with a
free space NA 0.4 objective. Lines shown here were patterned at
270.about.320 mW at 100 .mu.m/s.
[0030] Isolated lines and gratings were patterned. FIG. 3a,b shows
an AFM scan of an isolated HSQ line in a .about.50 nm film. It can
be seen that the individual lines consist of .about.550 nm width
trenches bounded by similar sized buildups. The trench depth is
.about.30 nm, illustrating that complete dewetting of the films,
while possible, is not necessary, making this a greyscale
patterning process.
[0031] A distinction between dewetting and ablation is the
displacement of material as opposed to its complete removal. Due to
this, the patterning behaviour of lines as they approached one
another was of considerable interest. FIG. 4 shows the AFM
evolution of the written gratings with three pattern periodicities
(2 .mu.m, 1 .mu.m, and 0.8 .mu.m) in PS (18 kDa) of thickness
.about.50 nm. As adjacent lines approach one another, they go
through a transition from the isolated line behavior to where the
ridges start decreasing in size. After this point, the ridges
become the relevant feature as opposed to the trenches as they are
now the high resolution feature. At certain spacings, a bifurcation
of the pattern occurs where the ridges adopt two line-to-line
spacings (in FIG. 4 for PS 0.8 .mu.m spacing, .about.780 .mu.m and
.about.880 .mu.m) and two line widths (in FIG. 4, .about.500 nm and
.about.750 nm). At lower spacings (not shown), another regime can
be seen where the smaller ridge disappears and the lines adopt a
single line width. The latter leads to periodicities disparate from
the patterning periodicity, due to patterning roughly every other
line. This indicates that the mechanism that leads to their
formation is more complex than the simple linear combinations of
multiple line patterns. This shows that developer free, ultrahigh
resolution patterning is feasible with both ubiquitous polymers and
small molecules without using a high numerical aperture or
multiphoton effects.
[0032] Dewetting of thin films as a positive tone process through
FLaSk heating of the substrate is a relatively nascent technique
and has only really begun to show its potential for
development-free positive tone patterning of 1D or (barring
overlap) 2D structures. As currently presented, it exists in two
distinct forms: (1) patterning of isolated trench-ridge lines near
the optical limit and (2) patterning of subwavelength lines by
overlapping the exposures. While capability (2) is more exotic,
capability (1) should not be diminished; submicron 2D DW in a
method that requires no developer step and only uses inexpensive,
commodity polymers, free space optics, and visible (subwatt) lasers
could be a potentially competitive process, especially for
industrial scale fabrication tools where price is a critical
concern. Ironically, the presence of the overlap effects that allow
for (2) is the major limitation of this technique: patterning any
feature wider than a single line or crossing another feature will
be complicated by the overlap effects. This could potentially be
addressed by changing the focus, which is an effective way to alter
spot size and programming intricate focus, power, position paths to
make desired features, not unlike how 3D laser printers currently
operate. This is a problem of optimization and software that could
be approached in future; however, one way to limit these effects is
to increase the resolution of patterning. This could be
accomplished via the usual methods of increasing the NA or
decreasing the wavelength, but the efficacy of such a strategy
would be limited by the coupled thermal effects. Moreover, any
increase in NA lowers the scalability by lowering the working
distance and field of view. Instead, it would be desirable to
increase the resolution by limiting the thermal spread, which is
possible, by the incorporation of solvent for evaporative cooling.
Another possible manipulation is the thermal or etching properties
of the utilized polymer. Finally, it would be desirable to be able
to use this positive tone process for liftoff rather than just
etching.
[0033] Swelling the film with solvent is one way to improve the
process. The effects of solvent in can be shown to limit the extent
of thermal excitation, thereby increasing thermal gradients for
enhanced patterning. In fact, the dewetting lines in the solvent
exposed PS-PDMS BCP possessed resolutions often much greater
(.about.200-600 nm) than those observed in the single PS-only
lines. Furthermore, the incorporation of solvent was observed to
smooth the surrounding ridges (FIG. 5), which could allow for
limited line-to-line interactions. In addition, the quality of
lines patterned in FIG. 5 were enhanced by the utilization of a
PS-brush which was observed to increase the uniformity of the
dewetting of the BCP.
[0034] The selection of polymer can have a large effect on the
final pattern generated by FLaSk dewetting. FIG. 6a shows the
effects on linewidth of the glass transition temperature as one
example. This information can be used in the other direction, i.e.
the shape of the line profile resulting from FLaSk dewetting can be
utilized to determine the properties of the dewetted film.
[0035] The thickness of the film will also affect the
antireflection aspects of the patterning with FIG. 6b showing the
dewetting of PS films at three different thicknesses which are at
select points of the antireflection curve in FIG. 2a. For example,
films patterned near the minimum reflectivity thickness are
relatively narrow and stable over large ranges of laser power.
[0036] Discussion to this point has referred to films of single
composition. This technique is not limited, however, to homogeneous
films. For example, FIG. 7 shows patterning in a film obtained from
a recycled polystyrene cup which was only purified by particulate
filtration, demonstrating the ability to utilize highly
polydisperse, waste polymers. Furthermore, the use of bilayers has
been shown to lead to other unique methods of patterning, such as
the evolution of chains of near-circular dots that emerge in a
stack of .about.50 nm PVP (630 kDa) on -50 nm PS (30 kDa) which
appear to vary in their frequency in the line with power (FIG. 8).
This represents a method of obtaining 2D patterns from a 1D line
writing process without even overlap effects. Another possible
modification is the incorporation of nanoparticles into the film.
FIG. 9 shows the pattern resulting from the etching of a PVP film
with PVP nanoparticles incorporated. At low power, the particles
are relatively unaffected by the dewetting, but at higher powers
they are burnt away. This indicates a method for both the exposure
of embedded particles for patterning and also a method for cleaning
of the patterned lines from unwanted particulate impurities.
[0037] The heated portions of the film, whether completely or
partially dewetted, undergo large thermal shears. These can, in
turn, induce other structural effects. For example, in the films
patterned by FLaSk of the PS-PDMS block copolymer, the
thermally-induced shear has also been observed to reside in
alignment of the microdomains of the PDMS (FIG. 10)..sup.18
[0038] The technology disclosed herein may be of interest to
companies that make 2D laser direct write platforms, as a possible
method to make a low cost system to market to customers that are
interested in batch processing at minimal cost per unit. A system
designed to do 2D patterning by a dewetting mechanism can reach
competitive resolutions with current systems without the need for
several key attributes: (1) photosensitive media, (2) post-exposure
baking, and (3) development. Step (1) has two distinct
implications: (1.i) Polymers for dewetting can be much cheaper than
photoresist polymers (lab grade polystyrene is .about.$ 0.20 per
gram (which is 2 wt % of the solution) and coating solvent is
.about.$ 0.05 per mL compared to .about.$2 per mL of a typical
photoresist) and (1.ii) lab facilities where photosensitive media
are handled often must be dark-room-like environments or exposure
limits must be enforced. Step (2) simply reduces the number of
steps. Step (3) can be a major cost consideration as labs where
development requires both (3.i) a large supply of fresh developer
and also (3.ii) the means to dispose of used developer. Removal of
this step could present a large reduction in both the chemical
costs and also the environmental footprint of a facility. Beyond
equipment companies, companies who do their own in-house
patterning, such as those who manufacture photomasks and stamps
could employ this method in a custom system to produce custom
grating structures in a much more affordable fashion. In addition,
as the specific morphology of feature depends on the polymer
parameter, this technique may be adapted as a form of metrology for
determination of polymer behaviors under high thermal gradients and
rapid heating.
[0039] The superscript numbers refer to the references listed
herein. The contents of all of these references are incorporated
herein by reference.
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